The present invention relates to an ophthalmological surgery apparatus and methods and more particularly to a laser surgical apparatus for sculpting a cornea using a diffractive optical element (DOE) and methods of using the laser surgical apparatus.
The human eye includes two focusing elements which are the cornea and the lens. The corneal tear/air interface is the major refracting surface in the eye with an average thickness refractive power of 48.8 diopters (D). The refractive power of the cornea primarily depends on the curvature of the anterior surface. In order to correct abnormal refractive conditions of the eye surgical alteration or sculpting of the shape or curvature of the cornea has been employed. Corneal sculpting involves the removal of external layers of the cornea that affects the radius of curvature of the cornea. Altering the radius of curvature of the cornea increases or decreases the dioptric power of the front surface of the cornea that corrects any abnormal refractive errors. In recent years considerable advancement in low wavelength lasers has opened frontiers in refractive surgery allowing for reshaping of the surface of the cornea by employing such lasers.
Many methods have been suggested for reprofiling of the cornea. One such suggested technique employs the use of a low wavelength excimer laser to perform controlled ablative photodecomposition. Although altering the curvature of the outer corneal surface has been effective in correcting spherical myopia, the limitation with current laser technology is the inadequacy of corrective surgery in aspheric, irregularly astigmatic corneal surfaces.
Some disclosures that relate generally to the sculpting of a patient's cornea include U.S. Pat. No. 4,911,711 entitled "Sculpting Apparatus for Correcting Curvature of the Cornea" which discusses using an ultraviolet laser for sculpting the cornea to achieve optical correction through a newly shaped anterior surface of the cornea. The apparatus subjects the laser beam to certain shaping and homogenizing operations prior to any attempt to specially characterize the beam for a particular sculpting procedure. In a preferred embodiment, the shaping and homogenizing operations present a tolerably homogeneous beam of enlarged dimension, so that specialty characterizing may proceed on a dimensional scale that is greater than the corresponding dimension of ultimate surgical delivery to the eye, thereby enabling greater control of the quality of specialty characterizing. Provision is made for selectively monitoring the quality of the homogeneity and/or the specially characterized beam, with further provision for automatic cutoff of a laser beam delivery to any eye in the event that quality is not within the predetermined limits of tolerance. Preferably, all beam shaping, homogenizing, and characterizing operations proceed in a controlled environment.
U.S. Pat. No. 4,994,058 entitled "Surface Shaping Using Lasers" issued to Anthony Raven et al discloses a laser system and masking apparatus for reprofiling surfaces, such as corneal surfaces. The system includes a laser and a mask disposed between the laser and the surface to be reprofiled, the mask providing a predefined profile of resistance to laser radiation, such that upon irradiation, part of the radiation is selectively absorbed and part is transmitted to the surface of the eye in accordance with the masked profile to selectively erode the surface. The masking apparatus disclosed is a mask that may be affixed to the surface of the eye or may include a support structure to support and position the mask above the surface. The resistance profile is stated as created by varying the thickness or the composition of the mask.
U.S. Pat. No. 4,732,148 discloses the use of ultraviolet laser radiation to control ablation of the cornea. The control of laser flux results from controlled change of projected laser-spot size in the course of a given treatment. The spot size ranges from a maximum which covers the entire area to be treated to a tolerable minimum diameter.
Although many methods have been used or are under investigation for reprofiling the surface of the cornea using laser radiation, these methods do not allow controlled variation in the output flux. Therefore, it would be advantageous to provide an apparatus that allows for manipulation and modification of the irradiated flux density profile over the entire area to be treated. Through the use of corneal topography information and diffraction theory a diffractive optical element (DOE) can be designed to control the output flux density profile of a sculpting beam. By employing a DOE to control the output flux density profile of a sculpting beam predictable and controllable corneal sculpting can be accomplished.